The invention relates to an antilock brake system for a vehicle having locked center four wheel drive (four wheel drive on demand). More particularly, the invention relates to a method for allowing the reference velocity to recover when the vehicle experiences four wheel drive axle oscillations. It also relates to methods for determining when the vehicle is in four wheel drive, and when such axle oscillations are present.
Automatic control of brake pressure in an ABS depends on accurate determination of the vehicle reference velocity, which is compared with the wheel speeds in order to determine slippage at the individual wheels. Since the wheel speeds provide important inputs for calculating the reference velocity, problems with reference stability arise after a negative .mu.-jump (transition to road surface having a lower coefficient of friction) and on very low .mu. surfaces in general.
In vehicles having a locked center (part time) four wheel drive (4WD) mode, the sums of the wheel velocities of the front wheels equals the sums of the wheel velocities at the rear wheels. This is because the front and rear axles are connected without any differential. When braking wheels independently, as ABS does, this may create an inequality between the front and rear axle speeds, causing wind-up in the drive train, i.e. torsional loading of the prop shafts, axles, CV joints, etc. When the brakes are released, unwinding may occur, causing "swinging" of the wheel speeds. If this wind-up and swinging are not prevented through strategic braking (or not braking), 4WD axle oscillations can arise. This problem does not arise in full time 4WD vehicles, because there is some type of differential between the front and rear axles; this allows the front and rear axles to turn at different speeds.
Particularly in the 4WD mode, vehicle reference velocity can be "lost" within 2-5 seconds after a negative .mu.-jump in spite of the input from an acceleration sensor. This is caused by late response of the acceleration sensor, the reluctance to use this sensor to its fullest extent (for safety reasons), and four wheel drive axle oscillations which are present in a locked-center 4WD vehicle. These axle oscillations cause the wheel speed sensors to see high wheel re-accelerations, which in turn disable prior mechanisms which support or flatten the reference velocity.
The prior art does not address the problem of recovering the reference velocity in a 4WD drive vehicle experiencing axle oscillations. U.S. Pat. No. 5,364,174 describes an ABS for a 4WD vehicle and particularly relates to supporting the reference speed by use of an auxiliary reference speed when one or more wheels becomes unstable. Problems due to wheel or axle oscillations which could interfere with the wheel speed signals are not recognized.
The prior art also does not address the problem of 4WD axle oscillations which arise in a vehicle having locked center 4WD. U.S. Pat. No. 4,317,105 discloses using an accelerometer on the suspension in order to detect wheel oscillations, which in turn indicate the condition of the shock absorbers.
U.S. Pat. No. 4,511,014 discloses a system for automatically switching from 2WD to 4WD, and recognizes 4WD problems including vibration of the vehicle body. There is no recognition of interference with the sensed wheel speeds.